Difference between revisions of "Almost-period"

A concept from the theory of almost-periodic functions (cf. Almost-periodic function); a generalization of the notion of a period. For a uniformly almost-periodic function $f(x)$, $-\infty<x<\infty$, a number $\tau=\tau_f(\epsilon)$ is called an $\epsilon$-almost-period of $f(x)$ if for all $x$,
$$|f(x+\tau)-f(x)|<\epsilon.$$
For generalized almost-periodic functions the concept of an almost-period is more complicated. For example, in the space $S_l^p$ an $\epsilon$-almost-period $\tau$ is defined by the inequality
$$D_{S_l^p}[f(x+\tau),f(x)]<\epsilon,$$
where $D_{S_l^p}[f,\phi]$ is the distance between $f(x)$ and $\phi(x)$ in the metric of $S_l^p$.
A set of almost-periods of a function $f(x)$ is said to be relatively dense if there is a number $L=L(\epsilon,f)>0$ such that every interval $(\alpha,\alpha+L)$ of the real line contains at least one number from this set. The concepts of uniformly almost-periodic functions and that of Stepanov almost-periodic functions may be defined by requiring the existence of relatively-dense sets of $\epsilon$-almost-periods for these functions.